1. Field of the Invention
The present invention relates to a circuit for driving a discharge load such as a spark plug, a discharge electrode of a combustor or the like. And more particularly it relates to a forward type circuit configuration which feeds to a discharge load a high voltage output obtained from a high voltage coil of a transformer in accordance with turn-on of a switching element actuated to switch on and off a d-c input supplied thereto through a low voltage coil of the transformer, whereby exact ignition can be effected in the discharge load without failure under the condition that the rise time is shortened and still the duration of high voltage application is set to be sufficiently long equivalently.
2. Description of the Prior Art
In the conventional systems relative to such discharge load driving circuit of the type mentioned, there are generally known a capacitor discharge ignition system (hereinafter referred to as CDI system) and a full transistor system utilizing flyback energy of a transformer. FIG. 3 shows a discharge load driving circuit of such CDI system, whrein there are included a d-c power source 1, a power switch 2, a transformer 3, a switching element 4 consisting of a thyristor or the like, a capacitor 5, a discharge load 6 consisting of a discharge electrode of a spark plug, a combustor or the like, a current limiting resistor 7, and a resistor 8 for protecting a power source. The transformer 3 has a low voltage coil 31 and a high voltage coil 32. The d-c power source 1, the switch 2 and the switching element 4 are connected in series to the low voltage coil 31, and the capacitor 5 is connected between the anode of the switching element 4 and the ground. The high voltage coil 32 is grounded at one end thereof while the discharge load 6 is connected to the other end thereof via the resistor 7.
When the d-c power source 1 is connected by closing the switch 2, the capacitor 5 is charged through the protective resistor 8 so that its terminal voltage is increased. And upon arrival of the terminal voltage of the capacitor 5 at a predetermined level, a terminal voltage signal is fed to a control electrode of the switching element 4, which is thereby turned on. When the switching element 4 is turned on, a high voltage is generated in the transformer 3 due to the resonance of its inductance L with the capacitance C of the capacitor 5. The high voltage thus generated is applied via the high voltage coil 32 of the transformer 3 to the discharge load 6 to consequently cause a discharge of the load 6.
FIG. 4 shows the waveform of the coil voltage obtained from the transformer 3 in the circuit of FIG. 3, wherein the high voltage has a duration T.sub.c starting from the power-on instant t.sub.o.
FIG. 5 shows a discharge load driving circuit of full transistor system. In this diagram, the same reference numerals as those used in the foregoing example of FIG. 3 denote corresponding components. The main circuit of a switching element 4 consisting of a transistor and so forth is inserted between one end of a low voltage coil 31 of a transformer 3 and the ground, and a pulse signal is fed from a driving circuit 9 to a control electrode of the switching element 4 to perform a switching operation. The polarity of the low voltage coil 31 and the high voltage coil 32 of the transformer 3 is so predetermined that, in accordance with turn-off of the switching element 4, a high voltage output is generated in the high voltage coil 32 by a release of the flyback energy.
When the switching element 4 is driven by the driving circuit 9 in a state where the switch 2 is closed to connect the power source 1, the exciting energy accumulated in the transformer 3 during the on-time of the switching element 4 is obtained as flyback energy from the high voltage coil 32 upon subsequent turn-off of the switching element 4 and then is applied to the discharge load 6, thereby generating a spark discharge in the load 6. FIG. 6 shows the waveform of the coil voltage obtained from the transformer 3 in this stage of operation.
However, there exist the following problems in the conventional discharge load driving circuits mentioned above.
(a) Problems in CDI system
Since a high voltage is generated by the resonance of the capacitance C of the capacitor 5 and the inductance L of the transformer 3, it is impossible to attain a sufficiently long duration Tc of the high voltage application. In the general CDI system, the duration T.sub.c is at most 100 .mu.s or so which is insufficient as a discharge duration for a spark plug or the like. Consequently there occurs deficiency of the discharge energy to bring about inadequate propagation of a flame, hence causing incomplete combustion.
Generally a charge time of 2 ms or so is necessary to raise the terminal voltage of the capacitor 5 up to a level required for turning on the switching element 4. Therefore it is difficult to increase the discharge energy by repeating such discharge operations.
(b) Problems in full transistor system
Although the duration T.sub.c is relatively long as 1 ms or so, the rise time Tr is prolonged as will be described below. In relation to the inductance L of the transformer 3 and the exciting current I, the exciting energy E accumulated in the transformer 3 during the on-time of the switching element 4 is expressed as EQU E=1/2.L.I.sup.2
The exciting energy E is released synchronously with turn-off of the switching element 4 and is applied to the discharge load 6 to discharge the same. For ensuring a predetermined amount of the exciting energy E, therefore, it is necessary that the inductance L of the transformer 3 be set above a certain value. Meanwhile, in relation to the inductance L and the distributed capacity C, the self-resonance frequency f of the transformer 3 is expressed as EQU f=1/2.pi..sqroot.LC
As is clear from the above two equations, if the inductance L is set to be sufficiently great to ensure the required exciting nergy E for driving the discharge load 6, the self-resonance frequency f is lowered while the rise time Tr is prolonged. Consequently, in case the surface of the spark plug constituting the discharge load 6 is soiled and the resistance value derived from such soil is not negligible, the operation is prone to become unstable as a spark discharge is not generated to eventually induce failure of ignition.